The Recovery of Microalgae Cells onto a Non-Porous Adsorbent

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Abstract

The threats of global warming attributed to fossil fuel combustion, combined with increasing energy demands and a growing population, have generated interests in diversifying the world energy mix. Biofuels from microalgae offer a sustainable renewable option and do not suffer the sustainability issues associated with early forms of bioenergy. However, research efforts of nearly 5 decades have not resulted in any significant gains and have motivated further investigation into novel techniques. The dilute nature of microalgae suspensions often requires dewatering and drying, which adds to energy intensity and costs associated with recovery processes. Curiously, the conventional recovery techniques do not consider the characteristic tendency of microalgae cells for surface attachment. This behaviour of cells, coupled with the discovery of a non-porous adsorbent material, NyexTM particles, has brought to the fore an exciting prospect. This has motivated the underpinning question behind this research; does the non-porous characteristic of the NyexTM particles presents an opportunity to recover microalgae cells from suspension using an adsorption technique?Using Chlamydomonas reinhardtii as a model microalgae strain, preliminary batch studies revealed a rapid recovery of the cells onto the NyexTM particles; nearly 90% recovery was attained within one minute, which depends on suspension concentration. At a correlation coefficient, R2 = 0.99, the Freundlich isotherm was found to give a better description of batch systems than the Langmuir isotherm, which suggests that cell coverage onto the NyexTM particles may not be a simple monolayer adsorption. Although a low adsorptive capacity of 0.55 mg/g was measured, the equilibrium parameter (1𝑛⁄) of about 0.6 was well within the range for favourable adsorption (i.e. 0 - 1). Further studies undertaken suggest that the recovery of cells could be driven by a hydrophobic-hydrophobic interaction, electrostatic forces of attraction and the flocculating behaviour of the NyexTM particles.Fixed bed studies showed that the lack of pores led to an early breakthrough. However, findings demonstrated that unlike most column studies, the bed capacity was a more valuable parameter to assess the column performance. Unexpectedly, depressed breakthrough curves, where bed exhaustion never attained Ct/C0 = 1.0, were observed. Nonetheless, the modified dose response (MDR) model was found to predict the experimental bed capacity to a greater degree of accuracy than other models. Furthermore, this research exploited the logistic features of the Bohart-Adams and the Clark models to adapt them to the experimental data. The adapted models significantly improved the accuracy of predictions with R2 values > 0.99 for the depressed breakthrough curves.The conductive nature of NyexTM particles was explored to electrochemically regenerate the adsorbent and reuse it to recover more cells. A current density of 32 mA.cm-2 was sufficient to inactivate the cells, regenerate the adsorbent and attain a maximum percentage recovery. Interestingly, scanning electron micrograph showed that the membranes of the adsorbed cells were ruptured, during NyexTM regeneration, potentially leading to lipid release. The maximum lipids extracted into a hexane solvent was estimated as 30 μg/mL at a current density of 64 mA.cm-2.Overall, the potential to recover microalgae cells onto a non-porous adsorbent has been demonstrated. The prospect of rupturing membranes of adsorbed cells offers the opportunity to use this technique to recover microalgae cells for potential biofuel applications. The results obtained from this research can serve as the impetus to further exploit this novel procedure. Future work should consider high lipid producing varieties of microalgae strains, develop a robust protocol to account for all forms of lipids released and undertake an energy and cost analysis to develop the technology further.

Keyword(s)

Adsorption; Electrochemical; Microalgae cells; Non-porous adsorbent

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